1. Field of the Invention
This invention relates to voice communications over distributed digital networks (e.g. networks which do not have a central station) such as local area networks and, more particularly, to conference call communications on such distributed digital networks.
2. Description of the Prior Art
Telephonic communications have been well known and in widespread use for many years. The interconnection arrangements which allow selective connection of telephone sets is complex and has developed from manually operated switchboards to elaborate central switching systems now in commercial use. The use of telephonic communication is central to the efficient operation of many offices, laboratories, businesses and the like. Further developments in other fields have also been adapted for use with telephonic communications systems, permitting communication of data other than voice over the telephone system, such as facsimile messages and data originating in digital form in data processing equipment. More recently, telephonic communication systems have begun adopting digital transmission techniques allowing improved noise immunity and greater data density on the existing communication links. The use of optical fibers for the communication link is also an excellent medium for transmission of numerous digital signals representing acoustic or voice signals. However, use of commercial digital voice transmission systems has generally been limited to long distance communications and analog to digital conversion is done at a central office which interfaces with the long distance links. Therefore the analog connection from the subscriber telephone set to the central station remains subject to noise and interference problems.
Similarly, development of data processing equipment has resulted in the use of local, wide and global area networks for transmission of digital data between data processing systems and which will be collectively referred to herein as distributed digital networks, allowing more rapid transmission than is possible over analog telecommunication links using modulator/demodulator devices (Modems) to convert digital signals into audio frequency tones for transmission. These distributed digital networks are also distinguished from telecommunication networks by the fact that they do not have a central station. Thus control of the network, as would otherwise be exerted by a central station, is "distributed".
A distributed digital network has several inherent advantages over a telecommunication network having a central station. Most importantly, a distributed digital network will have no single point of failure and communications may continue between functional stations even when one or more of the stations is non-functional. Further, the distributed digital network can be arranged to enhance security and reliability of communications between stations on the network, such as by encryption or the increase of resistance to interference. A distributed digital network need not be limited to subscriber stations on the network since inter-network or gateway interfaces can be provided to allow connection of the distributed digital network to another network, such as a commercial network.
When a conference call connection has been made in the past, whether using a network with a central station, as in a commercial telephone network, or a distributed network, it has been the practice to use a summing node to sum all the voice signals from all conference member nodes and to then retransmit the summed voice signals to each of the conference member nodes. Such a practice may have been advantageous in analog networks since transmission quality, signal level and other parameters of the analog signal subject to variation could be centrally corrected, insofar as possible, prior to summing of the signals to result in a conference call of improved overall quality. This practice has also been followed, however, in digital networks where the advantage is not as great due to the reduced need for correction of variation in incoming voice signals.
The presence of a summing node also subjects the network to several problems. Specifically, the summing node itself becomes a point at which a single failure can disrupt communication throughout the conference connection or throughout the network. Also, since the voice signal from any conference member node is returned to that conference member node as a portion of the summed signal and at a delay from the initial transmission which may or may not be small, the difficulty of echo suppression at the subscriber (e.g. conference member) node is increased.
Further, particularly as applied to a distributed digital network, the summation of all voice signals is complex and requires equipment at each subscriber station of a cost which makes implementation of conference calls on distributed networks prohibitive. This is true because the technique of digital summation of voice packets requires the equipment to be designed to accommodate the maximum number of possible conference call members at each station, while the network may be used at that capacity only infrequently. Moreover, summation of the voice packets, if plural speakers are, in fact, permitted, has required either synchronization between conference member nodes, which increases the amount of data which must be transmitted over the network, or by providing for summation of the voice packets as they arrive at the conference member nodes, which requires a high degree of circuit complexity to compensate for or otherwise accommodate the separate transmission delays corresponding to each respective conference member. This is so because of the random nature of voice spurts and silences which prevents prediction of when or if a voice packet will arrive from a particular conference member node, particularly when silence detection or voice-activated switching is employed to reduce noise and/or network traffic. Furthermore, when the distributed digital network is used at high traffic density, delays in the network transmission of voice packets become greater and even less predictable.